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Date & Time Guides

Clear rules, worked examples and the right tool for each calculation.

23 guides

Working dates

Calendar Days vs. Business Days: Choose the Right Count

Compare elapsed days, working days and holiday exclusions with practical deadline examples.

4 min readMethods & worked examples

Working dates

Working-Day Deadlines: Weekends, Holidays and Start Rules

Calculate a deadline using the correct working week, holiday coverage and start-date convention.

4 min readMethods & worked examples

Date intervals

Inclusive Date Counting: When to Include the End Date

Understand why the same two dates can produce different totals and how to state the counting convention.

4 min readMethods & worked examples

Date intervals

How to Calculate Days Between Dates Without Off-by-One Errors

Work through date intervals, signed results, complete weeks and calendar period breakdowns.

4 min readMethods & worked examples

Calendar rules

Leap Years and Month-End Dates: How Calendar Math Works

Learn the Gregorian leap-year rule and how month-end clamping affects date offsets and anniversaries.

4 min readMethods & worked examples

Calendar rules

ISO Week Numbers: Week 1, Week-Years and 53-Week Years

Find the ISO week-year near New Year and understand why January dates can belong to the preceding week-year.

4 min readMethods & worked examples

Calendar rules

Reading a Year Calendar: Leap Days, Weekdays and ISO Weeks

Inspect any supported Gregorian year and separate day totals from week numbering and holiday coverage.

4 min readMethods & worked examples

Calendar rules

Convert ISO Week Dates and Ordinal Days to Calendar Dates

Convert week-year/week/weekday values and year/day-number values correctly. Examples cover week 53, New Year boundaries, leap days and invalid inputs.

4 min readMethods & worked examples

Countdowns

Countdowns and Time Zones: Calendar Days vs. Exact Time

Choose the right time zone and understand why a day count can differ from a live timer.

4 min readMethods & worked examples

Recurring dates

How Dates Stay Current Every Year: Recurrences and Rollover

Understand how annual date rules keep calendars and countdowns current beyond a stored year.

4 min readMethods & worked examples

Recurring dates

Easter and Movable Holidays: Why the Date Changes Each Year

Understand Western Easter, its related dates and the limits of predicting local lunar observances.

4 min readMethods & worked examples

Date offsets

Adding and Subtracting Dates: Days, Weeks, Months and Years

Choose a calendar unit, understand month-end behavior and calculate a future or past date correctly.

4 min readMethods & worked examples

Personal milestones

Birthday and Anniversary Dates: Targets, Recurrences and Leap Days

Find an age milestone or anniversary while preserving the original date and a clear leap-day convention.

4 min readMethods & worked examples

Time zones

Planning Meetings Across Time Zones: Use One Shared Instant

Convert a meeting's UTC instant, check local dates and account for clock changes before sharing it.

3 min readMethods & worked examples

Time durations

Elapsed Time Calculations: Hours, Minutes, Seconds and Overnight Ranges

Calculate an exact duration and avoid confusing overnight times with calendar-day totals.

4 min readMethods & worked examples

Calendar planning

Recurring Dates: Build a Schedule Without Month-End Drift

Build daily, weekly, monthly and annual schedules from the original date. Handle missing month-end dates, fifth weekdays and leap-day anchors without drifting.

4 min readMethods & worked examples

Calendar planning

Quarters and Year Progress: Calendar Dates, Fiscal Starts and Percentages

Find quarter boundaries and measure calendar-period progress without fixed 90-day or 365-day shortcuts. Includes fiscal-start examples and leap-year comparisons.

4 min readMethods & worked examples

Date arithmetic

Date Ranges: Overlap, Shared Endpoints and Midpoints

Compare two date ranges, choose an endpoint convention and calculate a halfway point. Worked examples explain touching ranges, contained ranges and odd-length intervals.

4 min readMethods & worked examples

Time & time zones

Unix Timestamps vs Calendar Dates: Units, UTC and Conversion

Convert seconds and milliseconds without mixing up their units. Understand the Unix epoch, negative timestamps, UTC instants and date-only inputs.

4 min readMethods & worked examples

Time & time zones

Work Hours, Breaks and Overnight Shifts: Calculate Elapsed Time

Calculate a dated shift, deduct the break you enter and interpret decimal hours. Includes overnight examples, input validation and daylight-saving limitations.

4 min readMethods & worked examples

Time arithmetic

Add and Subtract Durations: Hours, Minutes, Seconds and Decimal Totals

Add or subtract elapsed amounts without treating minutes as decimals or wrapping at midnight. Includes carries, borrowing, negative totals and conversion checks.

4 min readMethods & worked examples

Working days

Weekend Patterns and Working Calendars: Count the Right Dates

Separate weekday labels from working-day eligibility. Compare weekend patterns, holiday scope, custom closures and inclusive endpoints with a reproducible example.

4 min readMethods & worked examples

Time and time zones

PST to EST: Dates, Daylight Saving and Meeting Times

Convert Pacific and Eastern times with the right source date, distinguish fixed offsets from local zones, and resolve skipped or repeated hours before sharing a meeting.

7 min readMethods & worked examples

Keep exploring

View the tool library

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Days Between Dates

Compare two dates with calendar days, complete weeks and weekday counts. Choose whether to include the end date.

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Business Days Calculator

Count working days using your weekend pattern and optional excluded dates. Public holidays are excluded only when entered.

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Day of the Week

Inspect a Gregorian date, leap-year rules, ISO week numbers and a printable month calendar.

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Time Zone Converter

Convert an exact UTC instant to another IANA time zone.

About the author

Navjeet Kamboj

Author & editor · Age Calculator Lab

Navjeet Kamboj is the author and editor of Age Calculator Lab. He focuses on making age, date, milestone and planning calculations easier to understand through clearly explained methods, practical examples and responsible limitations.

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